A method for correction of drift phenomena in an electronic force-measuring device is disclosed. A force-measuring device is disclosed that is suitable for performing the method, in particular a balance.
In electronic force-measuring devices, particularly in balances, there are a variety of component elements that have to meet stringent requirements in regard to their mechanical properties. For example, particularly important for the precision of the balance are load cells with their guiding mechanisms, coupling elements and pivot elements, or force/displacement transducers with appurtenant sensor devices, e.g. strain gauges connected in a measuring bridge circuit. In a force/displacement transducer the linearity of the relationship between force and displacement is important, wherein the aim is for reproducible elastic properties. As described in [1], EP 0 945 717 A1, the requirements are for the an elasticity, the creep behavior, and the (mechanical) hysteresis to be as small as possible. In addition, the elements are required to be corrosion-resistant and preferably non-magnetic.
Although significant improvements have been achieved in the properties that are relevant for the precision of a balance, undesirable drift phenomena remain an issue of concern, in particular the so-called creep behavior, i.e. a change in the measuring result within the time interval of a measurement.
Other properties of a balance which determine the accuracy of a measurement besides creep and hysteresis, namely linearity, freedom from eccentric load errors, repeatability, temperature stability, settling time of transient oscillations, and resolution, are described for example in [2], “Bauen Sie Ihre Qualität auf solidem Grund!” (Build your Quality on Solid Ground!), company publication, Mettler Toledo GmbH, January 2001.
Determining factors for the accuracy of a measurement in a balance are described in [3], “Wägefibel” (Weighing Primer), Mettler Toledo GmbH, April 2001. As discussed in this reference, the accuracy of a balance depends on physical influence factors such as the effects of heat, moisture absorption or moisture release, electrostatic or magnetic interaction with the environment. It is therefore important that the location where the balance is set up is chosen so that undesirable physical influences are avoided. Furthermore, the operation of the balance, for example the level adjustment and the correct procedure for putting the balance into operation, the arrangement of the draft-protection elements, the selection of an appropriate weighing container, and the positioning of the load on the weighing pan are of critical importance for the measurement accuracy. Thus, the influence factors and effects as described here determine the state of the balance, i.e. the conditions on which the measurement accuracy depends.
Some of these extraneous influence factors which affect the measurement accuracy, or measurement deviations that are caused by the properties and the behavior of the balance, can be compensated through regulation techniques. However, there are some unfavorable conditions of a balance which are caused for example by vibrations, or by air drafts in the absence of a draft shield, where the influence factors or the resultant measurement error cannot be corrected.
A balance in which the drift phenomena caused by creep are corrected by a compensation for the drift-related components is disclosed in [4], U.S. Pat. No. 4,691,290. In the method that is used in this balance, a representation of the measured load and the status of the creep are determined and combined with each other in order to arrive at a measurement value that corresponds to the applied load, wherein the creep-related error components are compensated.
Furthermore, according to [4] the mathematical representation of the status of the creep is determined as a function of time, of the load being measured, and of the creep status that was determined at an immediately preceding time, in order to take factors into account that affected the balance previously.
All of references [1] to [4] are hereby incorporated by reference herein in their entireties.
The creep status in this balance is calculated based on constants that were determined and stored in the initial adjustment of the balance. According to [4], these constants need to be determined individually for each balance, because there are in most cases variations between different balances.
In spite of the measures described above to improve the drift behavior of balances, it has been found that undesirable drift phenomena can still occur.